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Measurement of heterogeneity in low consistency pulp refining by comminution modeling Heymer, Jens Olaf

Abstract

In pulp refiners, pulp in suspension flows through an annulus between a rotor and stator having surfaces with bars which impose cyclic loading on the pulp during bar crossings. This process has long been known to be heterogeneous in its treatment of pulp. However, there is no method available to measure heterogeneity of treatment in operating refiners. The objective of this thesis was to develop such a method. The approach was based on applying comminution theory and measurements of fibre length before and after refining. Specifically, measurements of fibre shortening were used to obtain two key parameters in the comminution differential equation: the selection function and the breakage function. A procedure was employed to account for breakage at any point along a fibre length instead of an arbitrary point as in previous studies. The selection function was expressed as the product of two factors: a probability of impact and a probability that intensity of an impacted fibre exceeded the rupture intensity. These probabilities were expressed in terms of refiner variables. Values for the selection and breakage functions were obtained by regression fitting to fibre length data. These length distribution data were obtained for a range of operating conditions in various refiner types. Findings for predicted numbers of impacts and intensities were consistent with the findings of previous studies. The probabilities derived from the selection and breakage functions gave distributions of numbers and intensities of impacts. The findings showed, for example, that homogeneity of treatment is more dependent on numbers of impacts than on uniformity of intensity during impacts. A single number “homogeneity index” was proposed and defined as the product of the number of impacts on fibres having length-weighted average and the probability of intensity being within +/- 2kJ/kg of the mean divided by the specific energy. Using this definition, it was shown that homogeneity of refining treatment increased with flow rate through the refiner and decreased with increasing groove depth and consistency. It also showed the degree to which increased residence time of fibres in a refiner could compensate for a coarse bar pattern in attaining homogeneity of treatment.

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